Power amplifiers are in particular used for amplifying electro-acoustic signals. A power amplifier has the task of amplifying an audio signal to a level that, combined with sufficient current to move the coil, produces the desired acoustic level from the loudspeaker. Power amplifiers are capable of delivering larger amounts of undistorted power to produce the subjective acoustic levels demanded by the consumer.
Audio amplifiers are electronic amplifiers that work with audio frequencies, i.e. in a frequency range up to approximately 20 KHz. Applications of power amplifiers are instrument amplifiers and sound systems for home, automotive and public use. The soundcard in a personal computer contains several audio amplifiers as does every stereo sound system.
Power amplifiers can be appropriately distinguished by their output power. Most devices include more than one type of audio power amplifier
The key design parameters of power amplifiers are frequency response, gain, noise and distortion. These design parameters are interdependent, wherein desirable higher gains of the frequency response often lead to undesirable increases in noise and distortion.
Early audio amplifiers were based on vacuum tubes. Audio amplifiers based on transistors became practical with the availability of cheap transistors. Operational amplifiers which include transistors, can also be used for audio amplification, for instance in pre-amplifier stages and tone control circuits.
Conventional analogue power amplifiers suffer from low efficiency. This is critical since power amplifiers handle considerable amounts of power. Accordingly, conventional analogue power amplifiers are in general provided with heat sinks of extruded aluminium to cope with the heat development. Negative side effects of inefficient power amplification includes high volume, weight, cost and reliability problems. Moreover, conventional analogue power amplifiers have lower energy utilization.
To increase efficiency of amplification switching power stages have been proposed which are well-known in the state of the art. A conventional switching power amplifier comprises a modulator, a switching power stage and a demodulation filter to reconstitute the modulated signal. Such a switching power amplifier is also called a class-D-amplifier. The non-linearity of the switching power stage within a class-D-amplifier presents a significant impediment to maintain the modulator performance throughout the subsequent power conversion by the switching power stage.
FIG. 1 shows a pulse modulation amplifier (PMA) having a class-D-power stage according to the state of the art. The PMA-power amplifier comprises a pulse modulator, a switching power stage, a demodulation filter and an error correction block. The pulse modulator is based on analogue or digital pulse modulation technique. The switching power stage connected to the output of the pulse modulator is supplied with DC power and converts the pulse modulated input signal to power level. Subsequently, the amplified pulse modulated signal is fed to a demodulation filter to reconstitute the modulated signal. The error correction control unit is provided for compensating any errors that are introduced during the pulse modulation, the power amplification or the demodulation.
The pulse modulator is the core element of the PMA power amplifier as shown in FIG. 1. The most common analogue pulse modulation methods for PMA-amplifiers are PWM (Pulse Width Modulation) and PDM (Pulse Density Modulation).
FIG. 2 shows four basic pulse modulation techniques according to the state of the art, i.e. pulse amplitude modulation (PAM), pulse width modulation (PWM), pulse position modulation (PPM) and pulse density modulation (PDM).
Pulse width modulation PWM is different from PAM in that it provides the information in time whereas PAM provides it in the amplitude. Consequently, the information is coded into the time position of the transition within each switching interval.
Pulse position modulation (PPM) differs from PWM in that the value of each instantaneous sample of a modulating wave causes the variation of the position in time of a pulse relative to its non-modulated time of occurrence. Each pulse has an identical shape independent of the modulation depths.
Pulse density modulation (PDM) as shown in FIG. 2d is based on a unity pulse width, height and a constant time of occurrence for the pulse width in the switching period. The modulated parameter is the presence of the pulse. For each sample interval it is determined, if the pulse should be present or not, i.e. the density of the pulses is modulated.
Most conventional PMA-amplifier according to the state of the art as shown in FIG. 1 employ a PWM pulse modulator. Pulse width modulation (PWM) can be categorized in to major classes by the sampling method, i.e. a natural sampling PWM (NPWM) and an uniform sampling PWM (UPWM). The cited sampling method PWM can also be differentiated by the edge. The edge modulation may be single-sided or double-sided. The modulation of both edges doubles the information contained in the resulting pulse train although the pulse train frequency is the same. PWM is used to vary the total amount of power delivered to a load such as a loudspeaker without the losses which normally occur when the power source drops its output voltage through resistive means. In theory, the power conversion within a switching power amplification stage has almost 100% efficiency. Furthermore, the power conversion is in general perfectly linear and does not contribute distortion or noise. A real power amplification stage has a limited efficiency and contributes some distortion and noise.
The error correction control unit within a conventional PMA-power amplifier as shown in FIG. 1 compensates errors. The output of the pulse modulator generally contains three distinct signal elements, i.e. the modulated output signal, distortion signal components related to the modulated signal and a high-frequency signal. The high-frequency signal output by the pulse modulator is composed of discrete components related to the signal carrier, intermodulations between carrier and signal, and a shaped noise signal.
To eliminate the error sources related to the switching power stage and the demodulation filter while maintaining the demodulator performance a digital PMA-power amplifier according to the state of the art as shown in FIG. 3 has been described in WO 98/44626. As can be seen from FIG. 3, an edge delay correction unit is provided between the pulse modulator and the switching power stage. The output signal of the pulse modulator is fed to the edge delay correction unit which corrects or “pre-distorts” the pulse modulated signal to generate a compensated pulse signal, such that the non-ideal behaviour within the subsequent power conversion and demodulation are eliminated. This is carried out by means of pulse edge delays on each of the pulse edges controlled by an input control signal generated by a error processing unit within the switching power stage which amplifies the pulses coming from the pulse modulator.
The errors caused by the switching power stage can be divided into pulse timing errors PTE and pulse amplitude errors PAE.
The pulse timing errors PTE arise because the delays from turn-on to turn-off to actual transition at the output of the switching power stage are different in the turn-on and turn-off phase. Furthermore, rising and falling edges of the signals are not infinitely fast.
The pulse amplitude errors PAE can arise from noise caused by the power supply that feeds the switching power stage. Any power supply ripple or noise intermodulates with the modulated audio signal. Furthermore, power switches within the switching power stage have a finite impedance. Additional errors are caused by non-ideal modulation and non-ideal demodulation.
The demodulation filter connected to the output of the switching power stage introduces further distortion and increases the total output impedance and consequently changes in the load impedance which changes the frequency response.
In the PMA-power amplifier according to the state of the art, the error sources, i.e. the pulse amplitude errors PAE and the pulse timing errors PTE are corrected by intelligent pulse retiming as explained with reference to FIG. 4.
The edge delay correction unit of the conventional PMA-power amplifier as shown in FIG. 3 comprises an integrated circuit limiter and a comparator. The modulated output signal is applied to the edge delay correction unit which provides compensation for the PAE-errors and PTE-errors by delaying the individual pulse edges in response to a control signal generated by the error processing unit. The pulse edge correction will be performed by delaying a leading edge of a pulse or a trailing edge of a pulse or both edges of a pulse. FIG. 4 illustrates the functionality of a linear double edge delay correction unit.
FIG. 3 shows an implementation of a double-sided edge delay correction unit within the conventional PMA-amplifier according to the state of the art as described in WO 98/44626.
The PMA-power amplifier according to the state of the art as described in WO 98/44626 and as shown in FIG. 3 comprises some severe draw-backs. A first disadvantage is that the edge delay correction unit according to the state of the art as shown in FIG. 3 does only work with a conventional pulse width modulated signal (PWM) as shown in FIG. 2b and not with other pulse width modulated signals.
Furthermore, the PMA-power amplifier as shown in FIG. 3 has a linear feedback in the analogue signal domain. The output of the demodulation filter is fed back to the error processing unit which outputs the control signal νe to the comparator. This analogue feedback signal can cause an overdrive of the edge delay correction unit, i.e. the analogue feedback loop causes stability problems and is inclined to oscillation. Furthermore, the control range is very limited. As can be seen from FIG. 4 the signal edge of an integrated νe has a predefined slope and lasts for a comparatively short time. The control range for adjusting the delay corresponds to the time period of the signal edges of the integrated signal νe. Consequently, the control range for performing a controlled edge delay is very small.
Accordingly, it is the object of the present invention to provide a switching power amplifier and a method for amplifying a digital input signal which compensates errors caused by a switching power stage and a demodulation filter with a stable feedback loop.